About 40 yds across if you can. If not, as big as you can get it while still maintaining some cover around it. Clear it out so only a couple sticks are sticking out of the water, should look like a nice little open pothole to the birds. Don't make it perfectly round either, make it oblong or jagged around the edges so it doesn't look man-made to the birds.
Can you get back to it with a 4 wheeler? If so, drive around in circles to compact it down a bit so it will hold a little more water next year. And while you are there you might as well hook onto a couple good sized logs and drag them into your pothole. That will look very good to the birds next year!
If you can get it to hold water you should be golden. But again, I would suggest opening up the trees around it a bit (cut every other one down) so birds can see you from a ways off. Won't matter for local birds, but the migrators have to be able to see it from the river or they won't even know you are there.
Why do I shoot 3.5" for geese? Because they don't make a 4" yet!